The aim of the study was the evaluation of results of radiofrequency (RF) catheter ablation in relation to characteristics of regional endocardial potential morphology. 418 RF current deliveries in 26 patients with anterogradely conducting left-sided accessory pathways (AP) were investigated. A large regional atrial deflection (> 1/4 of ventricular potential) prior to RF discharge is a prerequisite for ablation success using the ventricular approach. A failing or extremely short (< 10 ms) isoelectric interval between atrial and ventricular deflections of the regional potential predicts a block of the AP. A persistent block can be expected if the regional electrogram contains an AP potential and the AP block occurs early (< 5 s) during onset of RF current.